1. Field of the Invention
The present invention relates to an interchangeable lens type imaging apparatus, and in particular, to an imaging apparatus to which a lens apparatus provided with a diaphragm for adjusting an amount of incident light is detachably attached.
2. Description of the Related Art
Conventionally, at the time of execution of focus detection by the contrast evaluation method in which a focus is detected based on contrast of an object image (i.e., contrast automatic focus (AF)), an exposure amount adjustment is performed in a manner suitable for the focus detection area prior to the AF to achieve an appropriate exposure amount in the focus detection area in the imaging plane. For example, Japanese Patent Application Laid-Open No. 2004-138970 discusses a relevant technique.
For example, when contrast AF is performed on an object in a backlight condition, execution of the exposure amount adjustment discussed in Japanese Patent Application Laid-Open No. 2004-138970 enables a further enhancement of the contrast of the object on which the focus detection is performed, thereby facilitating the focus detection.
Further, as a diaphragm constituted by a stepping motor and controlling an amount of incident light to an imaging optical system, conventionally, there is known a diaphragm device capable of finely controlling the aperture control speed of the diaphragm, as discussed in, for example, Japanese Patent Application Laid-Open No. 6-88985.
As discussed in Japanese Patent Application Laid-Open No. 6-88985, the control speed of the diaphragm aperture can be finely controlled by selectively switching a waveform of voltage to be applied to the stepping motor between rectangular waves and micro-step waves.
Conventionally, interchangeable lens type digital single lens reflex cameras used to be able to shoot only still images. Recently, however, there has been provided this type of camera that can shoot moving images as well.
Regarding the quality how an exposure amount is changed, it is desirable to maintain continuity of captured images to a certain degree by making a gradual change in the exposure amount in adjustment of it according to a change in the light amount of an object during moving image recording. However, in an exposure amount adjustment for focus detection, gradually changing the exposure amount to an appropriate exposure amount prolongs the time until a start of the AF, thereby prolonging the time until completion of the AF.
When an object image is displayed at an electronic viewfinder for the purpose of determining a composition at a stage before moving image recording (hereinafter referred to as “at the time of standby for moving image recording”), this is often accompanied by a composition change such as quick panning and a zoom adjustment, and, therefore, requires a certain level of followability of an exposure amount. In spite of this need, if an exposure amount is gradually adjusted at the time of standby for moving image recording because this occasion is handled as a kind of moving image recording, the followability of the exposure amount when a composition is changed is reduced.
On the other hand, a rapid change in the diaphragm aperture when the exposure amount is adjusted causes a rapid change in the depth of filed. Therefore, if the diaphragm aperture is suddenly changed when the exposure amount is adjusted during moving image recording, the continuity of the captured images is interrupted, resulting in deterioration of the image quality.
Despite commercialization of a digital single lens reflex camera that can record not only still images but also moving images, an attachable interchangeable lens therefor is designed for still image shooting. Therefore, in a conventional interchangeable lens, it is impossible to finely control the speed of controlling the aperture of the mounted diaphragm.